RFR: 8078641: MethodHandle.asTypeCache can retain classes from unloading

[Vladimir Ivanov]

On Wed, 25 Aug 2021 09:31:51 GMT, Vladimir Ivanov <vlivanov at openjdk.org> wrote:

> `MethodHandle.asTypeCache` keeps a strong reference to adapted `MethodHandle` and it can introduce a class loader leak through its `MethodType`.
> 
> Proposed fix introduces a 2-level cache (1 element each) where 1st level can only contain `MethodHandle`s which are guaranteed to not introduce any dependencies on new class loaders compared to the original `MethodHandle`. 2nd level is backed by a `SoftReference` and is used as a backup when the result of `MethodHandle.asType()` conversion can't populate the higher level cache.  
> 
> The fix is based on [the work](http://cr.openjdk.java.net/~plevart/jdk9-dev/MethodHandle.asTypeCacheLeak/) made by Peter Levart @plevart back in 2015.
> 
> Testing: tier1 - tier6

Thanks for the reviews, Paul and Mandy.

What do you think about the latest version?

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PR: https://git.openjdk.java.net/jdk/pull/5246